Coaching Centre
Chemical reactions review
A summary of the whole Chemical Reactions chapter, key words, and a mixed test covering balancing equations, acids and bases, combustion and corrosion, precipitation, displacement and decomposition, and chemistry in the body, industry and the environment.
Use this page to check the whole chapter. Read the summary, make sure you can explain every key word, then do the test without looking back.
Chapter summary
| Topic | What you need to know |
|---|---|
| 3A Equations | Formulas show elements and atom ratios; brackets multiply the group inside. Polyatomic ions (OH⁻, NO₃⁻, SO₄²⁻, CO₃²⁻, NH₄⁺). Prefixes for molecular compounds (CO, CO₂, N₂O₄). Reactants → products; coefficients, subscripts, state symbols (s, l, g, aq). Balance by changing coefficients only. Atoms are rearranged, so mass is conserved; gases escaping make mass seem to change. |
| 3B Acids and bases | Acids release H⁺; alkalis (soluble bases) release OH⁻. Indicators: litmus, universal, phenolphthalein, anthocyanins in red cabbage. pH scale is logarithmic (×10 per unit); 7 neutral. Strong = fully ionised (HCl, H₂SO₄, HNO₃); weak = partly ionised (ethanoic, citric, carbonic). Strength ≠ concentration. Alkaline ash in pituri. |
| 3C Acid reactions | Acid + base → salt + water (H⁺ + OH⁻ → H₂O, exothermic). Acid + metal → salt + hydrogen (pop test). Acid + carbonate → salt + water + CO₂ (limewater). Salts: chlorides, sulfates, nitrates, ethanoates. Titration: burette, indicator, end point; pH jumps at the equivalence point; weak acid equivalence point above 7. |
| 3D Combustion and corrosion | Oxidation. Metal oxides are basic; non-metal oxides acidic (SO₂, NOₓ → acid rain). Combustion of hydrocarbons is fast oxidation: complete → CO₂ + H₂O; limited oxygen → CO and soot (details in 4B). Rust = hydrated iron(III) oxide, needs water and oxygen, faster with salt. Protection: barriers, sacrificial zinc and magnesium, stainless steel. Charcoal pigments and radiocarbon dating. |
| 3E Precipitation | Ionic compounds dissolve into ions. Insoluble combination → precipitate (AB + CD → AD + CB). Spectator ions; ionic equations (Pb²⁺ + 2I⁻ → PbI₂). Solubility rules: Na⁺, K⁺, NH₄⁺, NO₃⁻ always soluble; most carbonates and hydroxides insoluble; AgCl, BaSO₄ insoluble. Tests for ions; water treatment. |
| 3F Displacement and decomposition | Activity series K Na Ca Mg Al (C) Zn Fe Sn Pb (H) Cu Ag Au. More reactive metal displaces less reactive (Zn + Cu²⁺ → Zn²⁺ + Cu), transferring electrons. Extraction: carbon for metals below carbon, electrolysis above. Decomposition by heat (CuCO₃, CaCO₃), electricity (water 2:1), light (H₂O₂). Reaction types. |
| 3G Chemistry in our world | Stomach HCl pH 1.5–3.5; antacids are weak bases; pancreas neutralises. Respiration, photosynthesis, fermentation. Blast furnace: C → CO₂ → CO; Fe₂O₃ + 3CO → 2Fe + 3CO₂; limestone forms slag. Green steel with hydrogen. Ocean acidification: CO₂ → carbonic acid, pH 8.2 → 8.1, less carbonate for shells and corals. |
Putting it together
Every reaction in this chapter is the same event seen from different angles: bonds break, atoms rearrange and new bonds form. Because atoms are only rearranged, equations must balance and mass is conserved. What the atoms do decides the type of reaction: H⁺ meeting OH⁻ is neutralisation, oppositely charged ions locking into a solid is precipitation, electrons passing from a more reactive metal to a less reactive one is displacement, a substance combining with oxygen is oxidation (fast as combustion, slow as corrosion), and one compound splitting up is decomposition. Recognising the type lets you predict the products, write the equation and choose a test for them. Put together, these ideas let chemists make steel, protect bridges, cure heartburn and predict what our carbon dioxide is doing to the oceans. Chapter 4 asks the next questions: how much energy each reaction releases, how fast it goes, and how chemists control both.
Key words
activity series · alkali · antacid · balanced equation · base · coefficient · combustion (complete, incomplete) · conservation of mass · corrosion · decomposition · displacement · electrolysis · endothermic · end point · equivalence point · exothermic · galvanising · indicator · ionic equation · ionise · neutralisation · ocean acidification · oxidation · pH · polyatomic ion · precipitate · sacrificial protection · salt · slag · spectator ion · state symbol · strong acid · subscript · synthesis · titration · weak acid
Test yourself
What are the coefficients that balance __ C₂H₆ + __ O₂ → __ CO₂ + __ H₂O?
4 C, 12 H and 14 O on each side.
0.01 mol/L hydrochloric acid has a pH of
[H⁺] = 10⁻², so pH 2.
Which reaction is a precipitation reaction?
Two solutions form an insoluble solid, Cu(OH)₂.
Iron is placed in silver nitrate solution. What happens?
Iron is above silver in the activity series.
Copper(II) carbonate is heated and the gas given off is bubbled through limewater. Which observations are expected?
CuCO₃ → CuO (black) + CO₂, and CO₂ turns limewater milky.
Which oxide forms an acidic solution when it dissolves in water?
Non-metal oxides are usually acidic; metal oxides are basic. SO₂ helps cause acid rain.
Which method protects steel even after its coating is scratched?
Zinc is more reactive and corrodes sacrificially.
Label the precipitation of lead(II) iodide.
Classify each reaction.
Order these metals from most to least reactive.
To balance an equation you change only the . Ions that take no part in a reaction are ions. Acid + carbonate gives a salt, water and . A solid that forms when two solutions are mixed is a . Rust is hydrated iron(III) .
A student adds 2.0 g of marble chips (calcium carbonate) to 50 mL of excess dilute hydrochloric acid in an open flask on a balance. (a) Write a balanced equation with state symbols. (b) Explain why the balance reading falls, and why this does not break the law of conservation of mass. (c) Describe a test that shows the gas is carbon dioxide. (d) The student repeats the experiment with magnesium ribbon instead of marble. Write a balanced equation and explain how the result of the gas test would differ, and what test should be used instead.
(a) CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). (b) The carbon dioxide gas escapes from the open flask into the air, so it is no longer on the balance and the reading falls. Mass is still conserved: atoms are only rearranged, so if the carbon dioxide were collected and weighed, the total mass of calcium chloride solution, water and carbon dioxide would equal the mass of the marble and acid at the start. 2.0 g of marble releases about 0.88 g of CO₂. (c) Bubble the gas through limewater (calcium hydroxide solution). Carbon dioxide turns it milky, because insoluble calcium carbonate forms. (d) Magnesium reacts with the acid to form a salt and hydrogen: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). The balance reading still falls as the hydrogen escapes, but hydrogen does not turn limewater milky. Instead, hold a lit splint at the mouth of a test tube of the gas: hydrogen burns with a squeaky pop.